1. Field of the Invention
The present invention relates to an apparatus for driving a photographic zoom lens in which at least first and second lens groups are disposed on the same optical axis and it is possible to rapidly perform a zooming operation for changing a focal length of the photographic zoom lens composed of a varifocal optical system having the first lens group as a focusing lens and perform a housing operation for moving the first and second lens groups to a housing position.
2. Description of the Related Art
Recently, cameras have been electronized and electrically operated. There are many proposals about a so-called varifocal camera which can set a plurality of focal lengths with respect to a photographing optical system by a single manual operation by driving a motor, etc. In a first proposed example of such a varifocal camera, in general, a cam ring has a cam groove and a cam pin is slidably fitted into the cam groove. The cam ring is rotated to move a lens frame having the cam pin therein in the direction of an optical axis so as to change the focal length of the photographing optical system.
Japanese Patent Application Laying Open (KOKAI) No. 60-79314 shows a second proposed example of the varifocal camera in which the lens frame is moved by one thread drive screw instead of the above cam ring in the optical axis direction.
In a third proposed example of the varifocal camera, an operation for housing the lens frame or a body tube, etc. is performed by the cam ring to make the camera compact.
The applicant of this application has proposed an apparatus for driving a photographic zoom lens in which at least first and second lens groups are disposed on the same optical axis and it is possible to perform a zooming operation for changing a focal length of the photographic zoom lens composed of a varifocal optical system having the first lens group as a focusing lens and perform a housing operation for moving the first and second lens groups to a housing position. This apparatus comprises a fixed frame fixed to a fixed section of an optical device such as a camera; a body tube frame movably supported within this fixed frame along the above optical axis; and a first group frame movably supported in parallel to the above optical axis within the body tube frame in a state in which the above first lens group is supported such that this first lens group can be focused. The above apparatus further comprises a moving amount detecting section for detecting a moving amount of this frist group frame with respect to the body tube frame in the optical axis direction; and a cam for zoom rotatably supported by the first group frame around the above optical axis and rotated by the operation of the moving amount detecting section corresponding to the above moving amount. The above apparatus further comprsies a second group frame movable along the above optical axis and restricted and operated by the operation of the above zooming cam in a state in which the second lens group is fixedly supported. The above apparatus further comprises a driving section such as a motor constituting a driving source for performing the above zooming and housing operations; and a transmission mechanical section for transmitting driving force from this driving section. The above apparatus further comprises at least three drive screws operated by this transmission mechanical section to respectively move the body tube frame and the first group frame by predetermined amounts. Members of the body tube frame and the first group frame disposed to be perpendicular to the above optical axis are respectively moved by the above three drive screws in parallel to this optical axis direction in a state in which these members are held to be approximately perpendicular to this optical axis. In the above apparatus, a zooming region for performing the above zooming operation, the movement of the above second group frame is restricted by the operation of the zooming cam. In a housing region for performing the above housing operation, the movement of the second group frame is not restricted by the operation of the zooming cam to hold a predetermined moving position. The above apparatus for driving the photographic zoom lens is proposed in Japanese Patent Application No. 62-326070.
In the first proposed example mentioned above, the construction of the driving apparatus is relatively simplified, but it is complicated to form the cam groove in the cam ring and high accuracy in processing is required so that the manufacturing cost of the apparatus is increased. Further, it is not possible to reduce the length of the cam ring in the optical axis direction so that the body tube cannot be made compact.
In the second proposed example, the lens frame is moved by the one thread drive screw instead of the above cam ring in the optical axis direction. In this case, driving force is concentrated onto a portion of the lens frame so that is is difficult to smoothly move the lenses. Further, the lens frame is inclined and the lenses cannot be held to be perpendicular to the optical axis. Therefore, it s not possible to give a predetermined optical performance of the lenses.
In the third proposed example, the operation for housing the lens frame or the body tube, etc. is performed by the cam ring. In this case, it is necessary to dispose a cam portion for moving the lens frame, etc. to the housing position in addition to a cam portion for performing the zooming operation with respect to the lens frame, etc. so that a pressure angle of the cam is increased. To solve this problem, it is necessary to increase the diameter of the above cam ring so that the size of the body tube is increased and it is difficult to make the apparatus compact.
In this case of the above-mentioned Japanese Patent Application proposed by the same applicant as that of this application, the above problems can be almost solved, but it is desirable to further simplify and make the transmission mechanical section and the drive screws compact and improve transmission efficiency of force so as to rapidly perform the zooming operation.
In this case, the moving amount of the first group frame with respect to the body tube frame in the optical axis direction is detected by a rack and a pinion to rotate the zooming cam. Further, the rotary speed of the pinion is suitably changed to transmit the rotary force to a crown gear integral with the zooming cam. In such a transmission system, a shift in zooming position is caused by an error in engagement between the rack and the gear, an error in engagement between a series of gears, accumulation of backlash of the three drive screws, etc.